A conventional differential amplifying circuit includes first and second transistors, first and second load resistors and a first constant current source. Such circuit is usually provided with an emitter follower circuit connected to an input of the amplifying circuit for increasing an input impedance thereof.
The first and second transistors are commonly connected at their emitters with the first constant current source, and commonly connected at their bases with emitters of first and second emitter follows transistors, respectively. The first and second load resistors are connected at first terminals to collectors of the first and second transistors, respectively, and commonly connected at second terminals to a positive terminal of a direct current power supply. The first and second emitter follower transistors are also commonly connected at their collectors to the positive terminal of the direct current power supply connected, at their base to a signal source in which positive and negative components are supplied to the bases of the first and second emitter follower transistors respectively, and connected at emitters to second and third constant current sources, respectively. The first through third constant current sources are commonly connected to a negative terminal of the direct current power supply.
In operation, an input signal is supplied differentially to the bases of the emitter follower transistors with bias voltage, and is amplified by the first and second transistors to be supplied to first and second output terminals provided at the collectors of the first and second transistors, respectively. In general, the outputs of the terminals may be amplified differentially in a differential amplifying circuit of the next stage.
According to the conventional differential amplifying circuit, however, there is a disadvantage in that the second and third constant current sources for driving the emitter follower transistors consume a considerable amount of power which is a multiple of the current flowing through the constant current sources and a voltage of the direct current power supply.
In case of the circuit used in a high frequency (1 GHz, for example) range, the constant current sources are required to produce currents sufficient for discharging a suspended capacity of the differential amplifying circuit and an input capacity of a differential amplifying circuit of the next stage, so that the currents may become 10 mA in some cases. In the case of the circuit used as a low noise amplifier, the constant current sources are required to produce currents sufficient for reducing shot noises to a predetermined level (it is known that the noise power level is inversely proportional to the current).
Further, a problem may occur, if the input bias current thereof is large. If the input signal source is an output of a magnetic head of a magnetic recording device such as a floppy disk drive, mis-writing operation may occur in reproducing mode, so that recorded signals may be unable to be reproduced, because of an offset of the input bias current. Therefore, the input bias current should be a low level. On the other hand, the constant current of the constant current source which supplies a bias current of the differential amplifying circuit is required to be a sufficiently large current in considering the gain, and noise and frequency characteristics.
Such a disadvantage may cause a short-life of batteries in a circuit driven by batteries or decrease of integration of LSIs to reduce a power consumption within an allowable loss thereof.